This invention relates to a module for optical communication having a semiconductor laser chip region and a modulator. The module for optical communication according to this invention is extremely useful when applied, for example, to an optical transmission module including a temperature controlled electroabsorption type optical modulator integrated laser. It converts electric signals into optical signals in the optical fiber communication.
In a modulator integrated semiconductor laser for use in optical communication (hereinafter referred to as a modulator integrated laser), it has been necessary to keep the chip temperature of the modulator integrated laser constant in order to stably keep the oscillation wavelength of the laser, optical output power, the form of the extinction curve and the chirping characteristics of the modulator in the semiconductor laser for long time even upon change of the environmental temperature or the like.
For instance, in an existent modulator integrated laser, a laser active layer and a modulator absorption layer are constituted with a multiple-quantum well (MQW) comprising InGaAsP (indium-gallium-arsenic-phosphorus) for the laser active layer. Accordingly, in view of the feature of the band structure, it results in a problem of lowering the optical power at high temperature and, at the same time, a problem in view of long time stability of the wavelength. Therefore, optical signals have been transmitted while setting the temperature of the semiconductor laser chip constant at a temperature of 30xc2x0 C. or at a temperature sufficiently lower than that.
Further, with an aim of efficient operation of optical networks and transmission modules, an optical modulator integrated laser having a wavelength variable function has been known recently. For example, this is described in a document (1): Japanese Patent Published Unexamined Patent Application No. Hei 4-72783 or in the recent document (2); IEEE Photonics Technology Letters, Volume 12, No. 3, p. 242. The wavelength variable function has been attained therein by controlling the temperature of the laser region. In the chip having the wavelength variable function, it is necessary that characteristics other than the oscillation wavelength of the laser, that is, the optical output power and the modulator performance can be kept stably for a long period of time also in a case where the temperature of the laser region is changed within a predetermined range, that is, a temperature range corresponding to the range in which the wavelength of the laser is intended to be changed.
Subjects will be described below for two cases of a modulator integrated laser having no wavelength variable function (hereinafter referred as a single channel modulator integrated laser) and a modulator integrated laser having a desired wavelength variable function (hereinafter referred to as a wavelength variable modulator integrated laser).
At first, an optical transmission module including a single channel modulator integrated laser is to be described. FIG. 12 is an example of a constitution for an optical transmitter 75 including a modulator integrated laser. The optical transmitter 75 has mounted therein an optical transmission module 74 (hereinafter referred to as a module) including a modulator integrated laser 1 according to this invention. In addition to the module, there are also mounted, in the optical transmitter 75, a multiplexer circuit for multiplexing a plurality of electric signals at a low bit rate inputted to the optical transmitter 75 into high bit rate signals, a modulator driver for increasing the amplitude of output signals from the multiplexer circuit such that the module 74 can be driven and a laser driving circuit for driving the three modules, a temperature controller circuit, a multiplexer (MUX) driving circuit and a driver driving circuit. In the example shown in FIG. 12, the module 74 and the modulator integrated laser housed therein have to be manufactured considering that the difference between the temperature of the module 74 and that of the outer wall of the module 74 is large (for example, 75xc2x0 C.). As shown in FIG. 1, as the difference between the temperature of the modulator integrated laser 1 and the temperature at the outer wall of the module 74 is larger, the consumption power of a Peltier cooler that controls the temperature of the modulator integrated laser increases abruptly. FIG. 1 is a graph showing an example of a relation between the difference of the case temperature to the chip temperature of a semiconductor laser chip, and the consumption power by the Peltier cooler.
Further, consumption power for other laser driving is relatively small as about 0.2 W. Accordingly, the consumption power for the entire module increases abruptly as the temperature difference increases.
However, when the temperature of the laser is made higher by using a multiple-quantum well structure constituted with InGaAsP for the laser active layer region, this results in the problem that (1) an optical power is lowered and (2) long-time stability for the oscillation wavelength can not be kept. Accordingly, the setting temperature for the module integrated laser has to be lower than 30xc2x0 C. On the other hand, in the transmitter, for example, as shown in FIG. 12, the consumption power for the modulator driver, the multiplexer (MUX) and the power supplier therefor is large, and the average temperature in the module transmitter is usually about 40xc2x0 C. or higher. Accordingly, if the setting temperature for the chip can be made higher than usual, it is possible to reduce the difference between the case temperature and the chip temperature of the optical transmission module and, the consumption power for the entire module can be decreased. Further, when it is intended to reduce the size of the optical transmitter (board) incorporated with the module or the optical transmission chip, the module as a heat generating source and other driving IC have to be disposed being close to each other. In this case, the chip environmental temperature will increase further.
In the existent modulator integrated semiconductor laser using InGaAsP for the laser active layer, the two problems described above hinder the rise of the setting temperature and decrease of the module consumption power.
Next, for making the oscillation wavelength of an optical modulator integrated laser variable, it is effective to control the wavelength by the temperature control for the laser region. In the document (2) above, temperature control is conducted not only for the laser region but also for the entire chip. This is a method of changing the temperature near the active layer of the laser thereby varying the oscillation wavelength of the distributed feedback type laser. However, since this results in a problem for the optical power level at high temperature and the long time stability of the oscillation wavelength as described above, it permits only the chip temperature of lower than 30xc2x0 C. as the operation condition capable of obtaining the longest wavelength right. Therefore, the chip temperature has to be lowered in order to make the wavelength variable range wider. Therefore, there has been a problem that the difference between the temperature of the module and that of the outer wall is large to increase the module consumption power. Further, in the optical modulator, the light wavelength suitable to transmission of optical digital signals for a long distance changes depending on the temperature and the variation coefficient is, for example, at 0.8 nm/xc2x0 C. On the other hand, the variation coefficient of the laser oscillation wavelength depending on the temperature is, for example, 0.1 nm/xc2x0 C. For keeping the modulation performance constant, it is necessary to keep the difference between the band gap wavelength of the modulator region and the oscillation wavelength of the laser substantially constant. For example, if the band gap wavelengths of both of them are excessively closer, it results in a problem of lowering the optical power and degrading the extinction ratio. However, when the temperature of the laser and the modulator are elevated while keeping them identical, since the change of the laser oscillation wavelength is merely xe2x85x9 for that of the band gap wavelength of the modulator, the laser oscillation wavelength approaches excessively to the optimal operation wavelength of the optical modulator.
In order to prevent this, the temperature for the modulator has to be controlled independently of the temperature for the laser region. In a case where most of the portions of the chip are controlled by a Peltier cooler (electronic temperature control element, electronic cooling (and heating) element) and the temperature near the laser active layer is controlled by a heater disposed on the chip, since the ratio of the temperature variation coefficient of the laser oscillation wavelength to the temperature variation coefficient of the band gap wavelength of the modulator region is 1:8, the amount of the wavelength variation for the laser region and the optimal amount for the wavelength variation of the modulator region can be made substantially identical, for example, by warming the laser region such that the temperature for the modulator rises by 1.25xc2x0 C. when the temperature for the laser region rises by 10xc2x0 C. For example, as shown in FIG. 13, this can be attained by properly designing the distance between the modulator and the laser when locating a heater 76 on the modulator integrated laser such that the heat conductivity from the heater 76 to the active laser 77 is 8 times as much as the heat conductivity from the heater 76 to the modulator region 78. There exists an upper limit for the temperature at which the laser 77 operates appropriately (for example, 30xc2x0 C.). On the other hand, for making the wavelength variable, the laser region has to be heated locally by the heater while keeping the temperature for most of the portions of the chip lower than the upper limit for the laser operation. As the temperature is relatively lower than the upper limit for the case temperature to be considered, the temperature for the entire chip as a reference before heating by the heater has to be lowered corresponding to the required range for the wavelength variation. For example, for obtaining a wavelength variable range of 4 nm at the upper limit value for the laser operation temperature of 30xc2x0 C., the chip temperature has to be at xe2x88x9210xc2x0 C. In this case, the shortest oscillation wavelength can be obtained. When the temperature for the laser region is changed to 30xc2x0 C. by heating with a heater while keeping the temperature for the entire chip at xe2x88x9210xc2x0 C., an oscillation wavelength longer by 4 nm can be obtained. In this case, the temperature for the entire chip is lower as much as by 85xc2x0 C. than the upper limit of 75xc2x0 C. for the case temperature. This difference increases as the upper limit for the laser operation temperature lowers to increase the module consumption power. As described above, in the laser constituted with an InGaAsP MQW, while the lowest chip setting temperature has to be lowered since the lowering of the output power at high temperature results in the problem. However, the consumption power increases as the case temperature of the optical transmission module is higher. The lowest temperature is often lower than the room temperature and the problem is more serious compared with the case of usual optical transmission module with no wavelength variable function.
The module for optical communication of this invention basically comprises at least an active region of a semiconductor laser, a modulator region for modulating the light from the active region of the semiconductor laser, a temperature control component for temperature control at least of the modulation region, in which the active layer of the semiconductor laser has a multiple-quantum well structure having at least two quaternary mixed crystal layers selected from the group consisting of quaternary compounds of In, Ga, Al and As and quaternary compounds of In, Ga, N and As, in which the temperature of at least the modulator region during operation can be set to 30xc2x0 C. or higher. When the module has the modulator which is integrated with the semiconductor laser chip, temperature setting therefor is often adapted such that the temperature for the active region of the semiconductor laser chip and the component, in the module, thermally in contact with the semiconductor laser chip for holding the active region of the semiconductor laser chip can be set to 30xc2x0 C. or higher. The active region of the semiconductor laser chip, specifically the active layer region thereof is an important region for the temperature control.
In this invention, it is important that the active region of the semiconductor laser is constituted with a multiple-quantum well structure comprising a quaternary mixed crystal layer selected from the group consisting of quaternary compounds of In, Ga, Al and As or quaternary mixed compounds of In, Ga, N and As. By the use of the quaternary compound semiconductor material containing Al, it is possible to ensure the optical power characteristic in a high temperature atmosphere. Alternatively, an N-containing compound semiconductor material may also provide a similar effect.
By using the quaternary compound semiconductor material of In, Ga, Al and As, it is possible to make the band offset value of the conduction band larger than the valence band offset value. Accordingly, overflow of injection current at high temperature is reduced. As described above, in this invention, decrease of the optical power at high temperature can be suppressed. As described above in this invention, it is extremely important to make the band offset value of the conduction band larger than the band offset value of the valence band.
Accordingly, when other compound semiconductor material is used as the semiconductor compound material constituting the quantum well structure, it is possible to extremely moderate the requirement of cooling the optical modulator in order to obtain a predetermined optical power. That is, the temperature for the modulator region can be set at a temperature higher than 30xc2x0 C.
Further, in this invention, the temperature control component usually used for the semiconductor chip region, for example, a thermoelectric cooler may be no more necessary. Of course, the feature of this invention can be attained also by using the usual temperature control component as described above. Also in this case, since the temperature can be set at a higher temperature than usual, for example, 31xc2x0 C. or 34xc2x0 C., the consumption power can be decreased effectively. In this specification, xe2x80x9cnot using cooling componentxe2x80x9d means not using such positive cooling component, cooling component with power consumption, specifically, an element resulting in power consumption, for example, a Peltier cooler. Accordingly, it does not mean to exclude the use, for example, of an air cooling component obtained by the structural consideration.
Furthermore, the temperature during operation of this optical module can be set at 35xc2x0 C. or higher. In this case, the consumption power can be reduced further. Also in this case, the feature of this invention can of course be practiced by using the usual temperature control component as those described above. Also in this case, since the temperature can be set higher than usual, the consumption power can be decrease extremely.
Heretofore, it has not even been thought that the operation at such high temperature is possible in a semiconductor laser device, among all, in a semiconductor light emitting device having on an optical modulator. This is attained for the first time in accordance with this invention.
Constitution for the quantum well structure by using the InGaAlAs series compound semiconductor or InGaNAs series compound semiconductor may be in accordance with a usual method.
The inventive idea of this invention is useful for a multi-active region semiconductor laser device, a semiconductor laser device having a plurality of oscillation wavelengths, and a semiconductor light emitting device having a semiconductor laser region and an optical modulation region. For example, an active region corresponding to a predetermined wavelength of DWDM (Dense Wavelength-Division Multiplexing) can be constituted as a plurality of chips, that is, as a multi-chip. Further, when the semiconductor laser region has a plurality of oscillation wavelengths, this invention is extremely useful.
Among all, this invention is useful for the semiconductor laser device in which a modulator is integrated. In this case, a semiconductor light emitting device having a plurality of active regions and synthesizing the lights therefrom in a multiplexer and modulating the light by an optical modulator is a typical embodiment.
This invention can also take an embodiment in which respective regions constituting a semiconductor light emitting device such as the semiconductor laser chip region and the modulator region or the multiplexer may be constituted as individual separate semiconductor chip regions, or as an embodiment in which such regions are constituted as a semiconductor chip integrated on one identical substrate.
As the modulator, a usual electroabsorption type optical modulator is useful.
In one of most typical examples of this invention. a temperature control component, for example, a heater is disposed near the active layer region of the semiconductor laser region, to enable control for the oscillation wavelength. Furthermore, in such an example, a temperature control component, for example, a heater is often disposed near the optical modulation absorption layer of the modulator.